1. Field of the Invention
The present invention relates generally to a mobile communication apparatus which can be implemented with a less number of components, and more particularly, to a transceiver which employs a direct conversion scheme suitable for larger scale of integration.
2. Description of the Related Art
With explosive popularization of mobile communication apparatus, requirements for a reduction in size and cost have been increased. For this reason, it is desired to eliminate VCO (voltage controlled oscillator), reduce the number of filters, and apply integrated circuits with a higher degree of integration. A prior art example of a transceiver which meets such requirements is described in K. Takikawa et. al., “RF Circuits Technique of Dual-Band Transceiver IC for GSM and DCS1800 applications,” IEEE 25th European Solid-State Circuits Conference Preprints pp. 278-281, 1999. The configuration of this transceiver is illustrated in FIG. 10A. The illustrated transceiver comprises an integrated circuit 1016, and other components 1001-1015 which are connected external to the integrated circuit 1016. The prior art example supports two frequency bands, i.e., 900 MHz band and 1.8 GHz band. Also, the transceiver employs a superheterodyne scheme for a receiver and an offset PLL scheme for a transmitter. The superheterodyne receiver requires two RF (high frequency) filters 1001, 1002 for suppressing out-of-band blocker signals; two image rejection filters 1003, 1004 for rejecting blocker signals in an image frequency band associated with mixing; and an IF (intermediate frequency) filter 1005 for filtering out blocker signals near a reception channel. The receiver also requires two local oscillators 1006, 1007 for supporting the two frequency bands, i.e., 900 MHz band and 1.8 GHz band.
A reception scheme which can reduce the number of externally connected components is a direct conversion scheme. A prior art example of a direct conversion receiver is described in Behzad Razavi, “A 900-MHz CMOS Direct Conversion Receiver,” IEEE Symposium on VLSI Circuits, pp. 113-114, 1997. The configuration of this receiver is illustrated in FIG. 10B. Since no image response exists in principle, the direct conversion scheme does not require an image rejection filter. Also, an IF filter is eliminated since it can be replaced by a filter integrated in an IC. In this prior art example, a VCO 1025 oscillates at a frequency twice an input frequency of the receiver which is in a range of 1850-1920 MHz. When this receiver is applied to GSM, DCS1800 dual band receiver, the VCO 1025 must oscillate in a range of 1850 to 1920 MHz (for GSM) and in a range of 3610 to 3760 MHz (for DCS1800). However, since it is difficult for a single VCO to cover these frequency bands, two VCOs are required.
A widely known drawback of the direct conversion receiver is a DC offset voltage. This is generated because an input signal to mixers 1019, 1020 is equal to a locally oscillated signal in frequency. For example, if the locally oscillated signal leaks into an input terminal for an input signal, locally oscillated signals are mutually multiplied to generate DC offset voltage. A prior art example of a scheme for canceling the DC offset voltage is described in Asad A. Abidi et. al., “Direct-Conversion Radio Transceivers for Digital Communications,” IEEE Journal of Solid-State Circuits, pp. 1399-1410, vol. 30, no. 12. December 1995. The configuration of this transceiver is illustrated in FIG. 11. An output DC offset voltage of a variable gain amplifier composed of variable gain amplifiers 1101, 1103, 1105 and low pass filters 1102, 1104 is detected by a digital signal processor (DSP) 1106. The DSP 1106 outputs a DC offset voltage cancel signal to an input of the variable gain amplifier 1101 based on the detected information.